Fix datasource config to remove stderr logs

Modify the datasource configuration to create default datasources
for types `db` and `mail`.

Before this change, `app.boot` was printing the following error:
{ name: 'AssertionError',
  actual: false,
  expected: true,
  operator: '==',
  message:
  'cannot autoAttach model "DataModel". No dataSource found of type db'
}
test: remove unneeded db cleanup

It is not necessary to remove Project/Model/DataSource definitions
created by previous tests, since the uniqueness validations are
correctly scoped now.
Project: implement `listAvailableConnectors`

The list is maintained in the file `available-connectors.json` for now.
PropertyDefinition: implement `availableTypes`

A list of types to offer when creating a new property.
Implement AclDefinition model.

Implement domain entity for working with model permissions.

Refactor Project and ModelDefinition to use this new entity when
loading and saving `models.json`.

Refactor Project > applyPermissions to use AclDefinition for adding
new permission records.

Add static value providers for accessType, role and permission.
Merge pull request #48 from strongloop/feature/generator-iteration2

Generator-related cleanup - the second iteration